Rocket motors are used to propel a variety of types of payloads at high speeds, such as in spacecraft propulsion and missiles. A casing of a conventional rocket motor is made of metal, a composite material, or a combination of metal and composite materials. During use and operation, an insulation protects the casing from thermal effects and erosive effects of particle streams generated by combustion of a propellant. The insulation is attached to an interior surface of the casing and is made from a composition that, upon curing, is capable of enduring high temperature gases and erosive particles produced while the propellant burns. A liner attaches the propellant to the insulation and to any noninsulated interior surface portions of the casing. One surface of the liner is in contact with the propellant and the other surface is in contact with the insulation or the casing. For effective operation of the rocket motor, the liner is securely attached to the insulation and, if applicable, the casing, and the liner and the propellant are securely attached to one another.
Conventional liners include a polymeric binder and a curing agent, such as a curative. Conventional propellants include a fuel, an oxidizer, a polymeric binder, and a curing agent, such as a curative. In order to be compatible with each other, precursor formulations of the liner and the propellant include similar ingredients, except that the liner typically does not include a fuel or an oxidizer. For instance, the liner and the propellant may include the same or a similar polymeric binder in that the polymer of the liner and the propellant may include the same functional group(s). The propellant is formulated to provide, during combustion, thrust for attaining rocket motor propulsion. During fabrication of the rocket motor, the insulation is prepared and secured to the casing. The liner is then prepared, applied to the insulation, and cured at an elevated temperature, which curing process may take up to a few days. The propellant is then prepared, applied to the liner, and cured. Since this process includes multiple process acts, the process is complex, time consuming, and expensive.
The combustion of the propellant generates extreme conditions within the casing. For example, temperatures inside the casing commonly reach 2,760° C. (5,000° F.). These conditions, in combination with a restrictive throat region of a nozzle passageway, create a high degree of gas turbulence within the casing and nozzle. In addition, gases produced during propellant combustion contain high-energy particles that, under a turbulent environment such as encountered in a rocket motor, erode the insulation. If the burning propellant penetrates the insulation and liner, the casing may melt or otherwise be compromised, causing failure of the rocket motor. A large number of rocket motor failures occur due to failure of the attachment between the insulation, the liner, and the propellant. For instance, an isocyanate curative of the propellant is known to diffuse into the liner, producing a soft layer at the liner-propellant interface that is prone to failure. At the time of applying the propellant, a majority of its curative has not reacted with the polymeric binder and, thus, is free to diffuse into the liner. Consequently, if diffusion occurs, the portion of the propellant proximal to the liner is deficient in curative, which produces a weak layer of propellant attached to the liner. Further, if a majority of the curative diffuses into the liner, uncured propellant is next to the liner. Moisture or contamination at any point in the process may also result in a weak attachment between the insulation and the liner or the liner and the propellant.
To compensate for the diffusion of the curative from the propellant and into the liner, an aziridine compound has been incorporated into liner formulations that include hydroxyl terminated polybutadiene (HTPB), carboxyl terminated polybutadiene (CTPB), or butadiene terpolymer (PBAN). The aziridine compound diffuses into the propellant and polymerizes in the presence of ammonium perchlorate to augment the HTPB, CTPB, or PBAN crosslinking. However, the use of the aziridine compound is only effective with liner formulations that include HTPB, CTPB, or PBAN and, thus, is not suitable for use with a broad range of liner and propellant formulation.
Another proposed solution to reduce the diffusion of the curative into the liner has been to use an isocyanate solution, which is applied to a surface of a cured liner or insulation. However, this leaves the isocyanate, a reactive material, exposed to environmental moisture, which can decrease the effectiveness of the isocyanate solution.
It would be desirable to improve the strength and reliability of attachment between the liner and the propellant to produce a more reliable rocket motor.